Power amplifier with distributed capacitor

ABSTRACT

A power amplifier integrated circuit includes a plurality of heterojunction bipolar transistors having a plurality of bases, a plurality of ballast resistors, and a capacitor. Each ballast resistor is connected between a base of the transistor and a DC node to which a DC voltage is applied. The capacitor is connected between an RF node, which supplies an RF signal, and the plurality of bases of the transistors. The capacitor provides a distinct path for the RF input signal having a substantially high capacitance, so that the RF input signal does not suffer significant signal loss.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a power amplifier integratedcircuit employed in wired or wireless system application, and morespecifically, to a power amplifier integrated circuit having adistributed capacitor.

[0003] 2. Description of the Prior Art

[0004] Power amplifier integrated circuits have been widely used indifferent kinds of wired or wireless system applications. Poweramplifier integrated circuits such as those employing heterojunctionbipolar transistors operate with elevated junction temperatures. Highjunction temperatures degrade device reliability and limit maximumcurrent density, and thus maximum power, of the power amplifier.

[0005] When operated at high power, power amplifier integrated circuitscan suffer thermal runaway where, because of emitter currentnon-uniformity or temperature profile non-uniformity, an emitter of thepower amplifier integrated circuit conducts an increasing amount ofcurrent until catastrophic device failure. Operating at high power alsoreduces the device life and mean time to failure (MTTF). Moreover, alarger device is mandated for a given application when a power amplifierintegrated circuit cannot operate at a required power.

[0006] Please refer to FIG. 1. FIG. 1 shows a prior art heterojunctionbipolar transistor power amplifier 10. Typically, the power amplifier 10includes a plurality of emitter fingers and base fingers. Accordingly,the power amplifier 10 is an equivalent circuit comprising transistors12 a-12 c. The power amplifier 10 further includes ballasting resistors14 a-14 c to stabilize the transistors 12 a-12 c when operating with ahigh current density, and bypass capacitors 16 a-16 c in parallel withthe ballasting resistors 14 a-14 c. An RF input signal and DC voltageare supplied through a common input node 18 and output is taken from thecollector nodes 20. The power amplifier 10 is further described byKhatibzadeh et al. in U.S. Pat. No. 5,321,279, which is included hereinby reference. While, the ballasting resistors 14 a-14 c provide devicestability, they also undesirably reduce the overall gain of the poweramplifier 10.

[0007]FIG. 2 shows a second prior art power amplifier as described byPratt in U.S. Pat. No. 5,629,648, which is included herein by reference.Similar to the power amplifier 10, the power amplifier 30 comprisestransistors 32 a-32 c and ballasting resistors 34 a-34 c. Bypasscapacitors 36 a-36 c are also provided. An RF signal can be applied at anode 38, a DC voltage can be applied at a node 39, and a correspondingoutput can be taken from collector nodes 40. However, use of capacitors36 a-36 c is inefficient and mandates an overly complicated layout.

[0008] Generally, the prior art power amplifier circuits provide poweramplifier stability at the cost of gain, and do so in an inefficientway. Moreover, layout of the prior art power amplifier integratedcircuits is inefficient.

SUMMARY OF INVENTION

[0009] It is therefore a primary objective of the present invention toprovide a power amplifier having a distributed capacitor to solve theproblems of the prior art.

[0010] According to one preferred embodiment of the present invention,the power amplifier includes a plurality of transistors. Each transistorhas a base and a ballast resistor having a first terminal and a secondterminal. The embodiment further comprises a DC node, an RF node, and acapacitor having a third terminal and a fourth terminal. The firstterminal of each ballast resistor is connected to a base of theplurality of bases, and the second terminal of each ballast resistor isconnected to the DC node. The third terminal of the capacitor isconnected to the RF node and the fourth terminal of the capacitor isconnected to the plurality of bases of the transistor.

[0011] According to the preferred embodiment, the transistor is aheterojunction bipolar transistor.

[0012] According to the preferred embodiment, the ballast resistors areset to maximize uniformity of temperature of a plurality of emitters ofthe transistor.

[0013] According to the preferred embodiment, the capacitor has asubstantially high capacitance so that an RF input signal applied at theRF node suffers low signal loss.

[0014] It is an advantage of the present invention that the capacitorprovides a distinct path for the RF input signal, such that the RF inputsignal does not suffer significant signal loss.

[0015] It is a further advantage of the present invention that thecapacitor supplies capacitance that is distributed to each base of theplurality of bases.

[0016] These and other objectives of the present invention will no doubtbecome obvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a circuit diagram of a power amplifier according to theprior art.

[0018]FIG. 2 is a circuit diagram of another prior art power amplifier.

[0019]FIG. 3 is a circuit diagram of a power amplifier according to thepreferred embodiment of the present invention.

[0020]FIG. 4 is a layout diagram of a power amplifier integrated circuitaccording to the preferred embodiment of the present invention.

DETAILED DESCRIPTION

[0021] Please refer to FIG. 3. FIG. 3 shows a power amplifier 50according to the preferred embodiment of the present invention. Thepower amplifier 50 comprises a plurality of heterojunction bipolartransistors 52 a-52 n, and each transistors 52 a-52 n has its ownemitter finger and base finger. The quantity of three transistors 52a-52 n illustrated in this drawing is not limiting, and more or fewerare acceptable according to the present invention. The power amplifier50 further includes ballasting resistors 54 a-54 n corresponding to eachtransistor 52 a-52 n to stabilize the power amplifier 50 when operatingwith a high current density or a high power. A capacitor 56 is providedto couple an RF signal from an RF node 58 to all the bases of thetransistors 52 a-52 n. The capacitance of the capacitor 56 is selectedto be a relatively high value considering the resistances of theresistors 54 a-54 n and the properties of the transistors 52 a-52 n, andas a result, the RF signal suffers substantially low loss. A node 59 isprovided to supply a DC voltage to the bases of the transistors 52 a-52n through the ballasting resistors 54 a-54 n. Output of the poweramplifier 50 can be taken from collector nodes 60.

[0022] When operating, a DC voltage is supplied at the node 59 and thusto the base of each transistor 52 a-52 n through the individualballasting resistors 54 a-54 n. The RF signal is provided to the node 58and is coupled to the base of each transistor 52 a-52 n through thecapacitor 56. Losses to the RF signal are minimized by the relativelyhigh capacitance of the capacitor 56, and by the fact that the path ofthe RF signal does not include the ballasting resistors 54 a-54 n.Amplified output of the RF signal is taken from the collector nodes 60.

[0023] Generally, the base ballasting resistors 54 a-54 n can be anysuitable source of resistance that can dissipate undesirable heat. Theselection of resistance values of the resistors 54 a-54 n is determinedwith respect to the layout of the power amplifier 50 to maximizeuniformity of temperature of emitters of the transistors 52 a-52 n. Thecapacitance of the capacitor 56 is selected as relatively high tominimize loss that the RF signal suffers.

[0024] It should be noted that the resistors 54 a-54 n connected inparallel might electrically equal one smaller resistor connected to thebases of all the transistors 52 a-52 n. However, it is preferable foreach transistor 52 a-52 n to have one resistor for thermal stability dueto different base-emitter voltages of each transistor 52 a-52 n.Respective resistors are suggested in this preferred embodiment.

[0025] In practical application, the power amplifier 50 is disposed onan integrated circuit. The heterojunction bipolar transistor asrepresented by transistors 52 a-52 n, the resistors 54 a-54 n, and thecapacitor 56 are all fabricated according to standard integrated circuitfabrication practices.

[0026] Please refer to FIG. 4. FIG. 4 shows a layout diagram of a poweramplifier integrated circuit 70 according to the preferred embodiment ofthe present invention. The power amplifier integrated circuit 70comprises heterojunction bipolar transistors having bases 72, emitters74, and collectors 76. A DC path 78 comprising ballasting resistorgroups 80 is provided to conduct a DC voltage applied at nodes 82 to thebases 72. An RF path 84 is provided for conducting an RF signal to becoupled to a capacitor 86. The capacitor 86 is made up of tworectangular regions of metallization 86 a, 86 b disposed on differentlayers of the integrated circuit 70. The power amplifier integratedcircuit 70 illustrates how the power amplifier 50 can be practicallyrealized in one preferred embodiment.

[0027] Individual resistors of the ballasting resistor groups 80 can bedesigned having resistances according to the specific design parametersof the power amplifier integrated circuit 70. Similarly, the tworectangular regions of metallization 86 a, 86 b can be designed toprovide the capacitor 86 with a suitable capacitance given relevantdesign considerations.

[0028] The layout of the power amplifier integrated circuit 70 as shownin FIG. 4 provides for efficient manufacture and improved thermalcharacteristics. Specifically, the layout allows the emitters 74 to bebumped periodically such layout is more conducive to positioningbackside vias in a manner providing for improved thermal dissipation ofexcess heat resulting from the operation of the power amplifierintegrated circuit 70. As a result, the power amplifier integratedcircuit 70 can handle more power/current than prior art designs.

[0029] Operation of the power amplifier integrated circuit 70 iseffectively the same as the previously described operation of the poweramplifier 50.

[0030] In contrast to the prior art, the present invention poweramplifier provides a capacitor that is distributed to a plurality ofbases of heterojunction bipolar transistors. The capacitance of thecapacitor can be set to minimize loss to an RF signal. Of equalimportance, the layout of the present invention power amplifierintegrated circuit offers improved thermal characteristics. The poweramplifier circuit according to the present invention is more efficientthan the prior art, and can reduce cost and fabrication time.

[0031] Those skilled in the art will readily observe that numerousmodifications and alterations of the device may be made while retainingthe teachings of the invention. Accordingly, the above disclosure shouldbe construed as limited only by the metes and bounds of the appendedclaims.

What is claimed is:
 1. A power amplifier integrated circuit comprising:a plurality of transistors each comprising a base; a plurality ofballast resistors corresponding to the plurality of transistors, eachballast resistor having a first terminal and a second terminal, thefirst terminal connected to the base of the corresponding transistor; aDC node to which the second terminals of the plurality of ballastresistors are connected; an RF node for supplying an RF input signal;and a capacitor having a third terminal and a fourth terminal, the thirdterminal connected to the RF node and the fourth terminal connected tothe plurality of bases.
 2. The power amplifier integrated circuit ofclaim 1 wherein the transistor is a heterojunction bipolar transistor.3. The power amplifier integrated circuit of claim 1 wherein thecapacitor has a substantially high capacitance so that the RF inputsignal applied at the RF node suffers low signal loss through the poweramplifier integrated circuit.
 4. The power amplifier integrated circuitof claim 1 wherein the capacitor comprises two regions of metallizationin two different layers of a semiconductor.
 5. The power amplifierintegrated circuit of claim 1 wherein the ballast resistors are set tomaximize uniformity of temperature of a plurality of emitters of thetransistor.
 6. The power amplifier integrated circuit of claim 1 being aportion of a wired or wireless system application.
 7. The poweramplifier integrated circuit of claim 1 being a portion of a mobiletelephone.